The invention relates to a process for the production of high-tenacity technical-grade yarns, particularly of polyamide and polyester, having a low reference elongation, by spin-drawing, in which the filaments extruded from a spinneret are cooled in a cooling zone by being exposed to a stream of air, are passed over a preparation device, and then passed directly over several sets of rolls to be drawn, in at least one draw field, between at least two of the sets of rolls. The filaments are subjected to a temperature of at least 160.degree. C. in at least one set of rolls and, finally, are passed over a set of let down rolls prior to being wound up at a speed of at least 2,200 meters/min.
In order to use the yarns for straps, belts and layer webs for textile fabrics, the heat shrinkage of the yarns should be low and, at the same time, the lowest possible reference elongation should be sought.
Feed yarns made from polyamide or polyester will undergo a change in textural condition during the various process stages. In the article, "Spinning Process and Crystal Structure of Perlon," Angewandte Chemie, Vol. 74, 1962, No. 13, p. 566, it is noted that a yarn drawn directly during the spinning operation passes through crystallization stages that are different from those of a yarn made on a drawing machine from staple stock.
Furthermore, at higher production speeds, a uniform temperature transfer to the yarn becomes more difficult. Higher speeds also translate into shorter contact times between the yarn and the heating systems, so that the heating of the yarn to predetermined temperatures becomes more of a problem. However, it is the temperature and structural properties which largely determine the attainable qualitites of textile yarn.
The technical and economical value of the spin-draw process, however, can be rated as positive only if, at the same time, the textile yarn qualities are not impaired and even improved.
From German published patent application No. 1,435,467, a process for spin-drawing of polyester (PES) at speeds of 1000 to 4000 m/min. is known. The process disclosed therein utilizes a temperature treatment of the filaments at predetermined residence times prior to the drawing operation. However, no process parameters for drawing and thermal stabilizing, such as temperature control and yarn tensions, are disclosed. In applying the teachings of this reference, low reference elongation values were generally obtained in combination with high tenacities and low elongation or low shrinkage values.
U.S. Pat. No. 3,452,131 and German published patent application No. 1,912,299 disclose processes for the spin-drawing of polyamide filaments in which additional draw means are employed. In the case of a steam jet, there is a danger of simultaneous wetting or moistening because of the low yarn temperature, a fact which is well known, and which has a substantial influence on the glass transition point. At high speeds, i.e., short residence times, this expedient may lead to complications in the drawing process. The use of a stationary draw pin is known from conventional methods, but is recommended only for moderate speeds. At high speed production, the friction is too strong causing the increase in temperature of the pin to be uncontrollable. Both of these references fail to make any recommendations as to how low reference elongations might be attained.
If, in spin-drawing, the cooled filaments are drawn at high speeds on stationary draw means, and if, for the purpose of increasing productivity, more than one strand of filaments is processed at one station, there is danger of non-uniform draw action, particularly among the yarns at different stations.
U.S. Pat. No. 3,790,995 discloses a process for spin-drawing of polyester at speeds of at least 1800 m/min, in which the feed and draw rolls are provided with a surface roughness to allow slippage of the continuous filaments over a number of wraps on the rolls. Such filament slipping leads to a high degree of friction at high production speeds, and the frictional heat generated causes an uncontrollable increase in temperature of the rolls. No teaching can be derived from the given parameters of this reference as to how to achieve a low reference elongation.
U.S. Pat. No. 4,003,974 discloses a process for spin-drawing of polyester, in which the filaments are thermally stabilized or "set" at 225.degree.-250.degree. C., and are allowed to relax at a tension of 0.09-0.15 g/den, with a speed given, by way of example, as 1.829 m/min. It does not teach an operation at higher speeds. The given temperatures could not be further increased because temperatures in the range of the polymer melting point cause the filaments to stick to the rolls. An increase in the residence time of the filaments on heated rolls has limits for mechanical reasons. Relaxation tensions of the given range generally cause a shrinkage of the filament to an extent which necessarily is offset by a high and undesired reference elongation. As relaxation tensions of less than or equal to 0.2 g/dtex are applied to the heated, thermally set filaments, either between the heated draw rolls, or between the heated relaxation roll and the windup unit operating at different speeds, a severe yarn shrinkage occurs which, while yielding a lower heat shrinkage value, also results in a high reference elongation value. In the process disclosed, the heated, thermally fixed yarn is passed over the unheated relaxation or let down roll onto the windup or, if the last named roll was used for the thermal setting operation, directly onto the windup. The yarn is not allowed to cool off sufficiently fast and, therefore, has a tendency to shrink while under low tension. The result is an increase in the reference elongation. This becomes more critical with higher production speeds, and, thus, lower residence times of the yarn.